Part 15 (1/2)
(5) The process of perception.”
It is not our task to examine the first four elements. In order
clearly to understand the variety of perception, we have to deal with the last only. I once tried to explain this by means of the phenomenon of instantaneous photographs (cinematographs). If we examine one such representing an instant in some quick movement, we will a.s.sert that we never could have perceived it in the movement itself. This indicates that our vision is slower than that of the photographic apparatus, and hence, that we do not apprehend the smallest particular conditions, but that we each time unconsciously compound a group of the smallest conditions and construct in that way the so-called instantaneous impressions. If we are to compound a great series of instantaneous impressions in one galloping step, we must have condensed and compounded a number of them in order to get the image that we see with our eyes as instantaneous. We may therefore say that the least instantaneous image we ever see with our eyes contains many parts which only the photographic apparatus can grasp. Suppose we call these particular instances a, b, c, d, e, f, g, h, i, j, k, l, m; it is self-evident that the manner of their composition must vary with each individual. One man may compound his elements in groups of three: a, b, c,-d, e, f,-g, h, i, etc.; another may proceed in dyads: a, b,-c, d,-e, f,-g, h,-etc.; a third may have seen an un.o.bservable instant later, but constructs his image like the first man: b, c, d,-l, m, n, etc.; a fourth works slowly and rather inaccurately, getting: a, c, d,-f, h, i,-etc. Such variations multiply, and when various observers of the same event describe it they do it according to their different characteristics. And the differences may be tremendous. Subst.i.tute numerals for letters and the thing becomes clear. The relative slowness of our apprehension of visual elements has the other consequence that we interpolate objects in the lacun
and whether the difficulty might not really lie in the imperfection of our own sensory processes. This involves partly what Liebmann has called ”anthropocentric vision,” i. e., seeing with man as the center of things. Liebmann further a.s.serts, ”that we see things only in perspective sizes, i. e., only from an angle of vision varying with their approach, withdrawal and change of position, but in no sense as definite cubical, linear, or surface sizes. The apparent size of an object we call an angle of vision at a certain distance. But, what indeed is the different, true size? We know only relations of magnitude.” This description is important when we are dealing with testimony concerning size. It seems obvious that each witness who speaks of size is to be asked whence he had observed it, but at the same time a great many unexpected errors occur, especially when what is involved is the determination of the size of an object in the same plane. One need only to recall the meeting of railway tracks, streets, alleys, etc., and to remember how different in size, according to the point of view of the witness, various objects in such places must appear. Everybody knows that distant things seem smaller than near ones, but almost n.o.body knows what the difference amounts to. For examples see Lotze, ”Medical Psychology,” Leipzig, 1852.
In addition we often think that the clearness of an object represents its distance and suppose that the first alone determines the latter. But the distinctness of objects, i. e., the perceptibility of a light-impression, depends also upon the absolute brightness and the differences in brightness. The latter is more important than is supposed. Try to determine how far away you can see a key-hole when the wall containing the door is in the shadow, and when there is a window opposite the key-hole. A dark object of the size of a key-hole will not be visible at one hundredth of the distance at which the key-hole is perceived. Moreover, the difference in intensity is not alone in consideration; the intensity of the object *with regard to its background has yet to be considered. Aubert has shown that the accuracy of the distinction is the same when a square of white paper is looked at from an angle of 18”, and when conversely a square of black paper on white background is looked at from an angle of 85”. ”When we put a gray paper in the suns.h.i.+ne, it may become objectively brighter than white paper in shadow. But this does not prevent us from knowing one as gray and the other as pure white. We separate the color of the object from the intensity of the incident light.” But this is not always so simple, inasmuch as
we know in the case in hand which paper is gray and which white, which is in the sunlight and which in the shadow. But if these facts are not known mistakes often occur so that a man dressed in dark clothes but in full light will be described as wearing lighter clothes than one who wears light clothes in the shadow.
Differences of illumination reveal a number of phenomena difficult to explain. Fechner calls attention to the appearance of stars: ”At night everybody sees the stars, in daylight not even Sirius or Jupiter is seen. Yet the absolute difference between those places in the heavens where the stars are and the environing places is just as great as in the night-there is only an increase in illumination.” Of still greater importance to us is the circ.u.mstance noted but not explained by Bernstein. If, in daylight, we look into a bas.e.m.e.nt room from outside, we can perceive nothing, almost; everything is dark, even the windows appear black. But in the evening, if the room is ever so slightly illuminated, and we look into it from outside, we can see even small articles distinctly. Yet there was much intenser light in the room in question during the day than the single illumination of the night could have provided. Hence, it is a.s.serted, the difference in this case is a standard one. In open day the eye is accustomed to the dominating brightness of daylight, beside which the subdued illumination of the room seems relatively dark. But in the evening one is in the dark, and hence even the little light of a single candle is enough to enable one to see. That this explanation is untrue is shown by the fact that the phenomenon is not regulated even when the circ.u.mstances in question are made identical. If, for example, you approach the window in daylight with your eyes shut, lean your forehead against the pane and shut out the light on the sides with your hands, and then open your eyes, you see as little in the room as when you looked into it without performing this ceremony. So again, if during the night you gazed at some near-by gas lamp and then glanced into the room, there is only a few moments' indistinctness at most, after that the single candle is enough. The reason, then, must be different from the a.s.signed one-but whatever it is, we need only to maintain that immediate judgment concerning numerous cases involving situations of this kind would be overhasty. It is often said that a witness was able to see this or that under such and such illumination, or that he was unable to see it, although he denies his ability or inability. The only solution of such contradictions is an experiment. The attempt must be made either by the judge or some reliable
third person, to discover whether, under the same conditions of illumination, anything could be seen at the place in question or not.
As to *what may be seen in the distance, experiment again, is the best judge. The human eye is so very different in each man that even the acute examination into what is known of the visual image of the Pleiades shows that the *average visual capacity of cla.s.sic periods is no different from our own, but still that there was great variety in visual capacity. What enormous visual power is attributed to half-civilized and barbarous peoples, especially Indians, Esquimos, etc.! Likewise among our own people there are hunters, mountain guides, etc., who can see so clearly in the distance that mere stories about it might be fables. In the Bosnian campaign of 1878 we had a soldier who in numerous cases of our great need to know the enemy's position in the distance could distinguish it with greater accuracy than we with our good field-gla.s.ses. He was the son of a coal-miner in the Styrian mountains, and rather a fool. Incidentally it may be added that he had an incredible, almost animal power of orientation.
As we know little concerning far-sightedness, so also we are unable to define what near-sighted people can see. Inasmuch as their vision does not carry, they are compelled to make intellectual supplementations. They observe the form, action, and clothes of people more accurately than sharp-eyed persons, and hence recognize acquaintances at a greater distance than the latter. Therefore, before an a.s.sertion of a short-sighted man is doubted an experiment should be made, or at least another trustworthy short-sighted person should be asked for his opinion.
The background of objects, their movement and form have decided effects on the difference in visual perception. It is an ancient observation that lengthy objects like poles, wires, etc., are visible at incomparably greater distances than, e. g., squares of the same length. In examination it has been shown that the boundary of accurate perception can hardly be determined. I know a place where under favorable illumination taut, white and very thin telephone wires may be seen at a distance of more than a kilometer. And this demands a very small angle of vision.
Humboldt calls attention to the large number of ”optical fables.” He a.s.sures us that it is certainly untrue that the stars may be seen in daylight from a deep well, from mines, or high mountains, although this has been repeatedly affirmed since Aristotle.
The explanation of our power to see very thin, long objects at
a very great distance, is not our affair, but is of importance because it serves to explain a number of similar phenomena spoken of by witnesses. We have either incorrectly to deny things we do not understand, or we have to accept a good deal that is deniable. We will start, therefore, with the well-known fact that a point seen for a considerable time may easily disappear from perception. This has been studied by Helmholtz and others, and he has shown how difficult it is to keep a point within the field of vision for only ten or twenty minutes. Aubert examines older studies of the matter and concludes that this disappearance or confusion of an object is peripheral, but that fixation of a small object is always difficult. If we fix a distant point it is disappearing at every instant so that an accurate perception is not possible; if however we fix upon a long, thin body, e. g., a wire, it is unnecessary to fix a single point and we may see the object with a wandering eye, hence more clearly.
Helmholtz adds that weakly objective images disappear like a wet spot on warm tin, at the moment a single point is fixed, as does e. g., a landscape seen at night. This last acute observation is the basis of many a testimony concerning the sudden disappearance of an object at night. It has helped me in many an examination, and always to advantage.
In this connection the over-estimation of the moon's illuminating power is not to be forgotten. According to Helmholtz the power of the full moon is not more than that of a candle twelve feet away. And how much people claim to have seen by moonlight! Dr. Vincent[1] says that a man may be recognized during the first quarter at from two to six meters, at full moon at from seven to ten meters, and at the brightest full moon, an intimate may be recognized at from fifteen to sixteen meters. This is approximately correct and indicates how much moonlight is over-estimated.
[1] Vincent: Trait'> de M'>decine l'>gale de L'>grand du Saule.
In addition to the natural differences of sight there are also those artificially created. How much we may help ourselves by skilful distinctions, we can recognize in the well-known and frequently- mentioned business of reading a confused handwriting. We aim to weaken our sense-perception in favor of our imagination, i. e. so to reduce the clearness of the former as to be able to test upon it in some degree a larger number of images. We hold the MS. away from us, look at it askant, with contracted eyebrows, in different lights, and finally we read it. Again, the converse occurs. If we have seen something with a magnifying gla.s.s we later recognize
details without its help. Definite conditions may bring to light very great distinctions. A body close to the face or in the middle distance looks different according as one eye or both be used in examining it. This is an old story and explains the queer descriptions we receive of such objects as weapons and the like, which were suddenly held before the face of the deponent. In cases of murderous a.s.sault it is certain that most uncanny stories are told, later explained by fear or total confusion or intentional dishonesty, but really to be explained by nothing more than actual optical processes.
I do not believe that binocular vision is of much importance in the law; I know of no case in ordinary vision where it matters whether one or both eyes have been used. It is correct to a.s.sert that one side or the other of a vertically held hand will be clearer if, before looking at it with both eyes, you look at it with one or the other, but this makes little difference to our purpose. It must be maintained that a part of what we see is seen with one eye only,-if, e. g., I look at the sky and cover one eye with my hand, a certain portion of the heaven disappears, but I observe no alteration in the remaining portion. When I cover the other eye, other stars disappear. Therefore, in binocular vision certain things are seen with one eye only. This may be of importance when an effect has been observed first with both eyes, then with one; raising the question of the difference in observation-but such a question is rare.
There are two additional things to consider. The first is the problem of the influence of custom on increasing visual power in darkness. This power is as a rule undervalued. No animal, naturally, can see anything in complete darkness. But it is almost unbelievable how much can be seen with a very little light. Here again, prisoners tell numerous stories concerning their vision in subterranean prisons. One saw so well as to be able to throw seven needles about the cell and then to find them again. Another, the naturalist Quatrem'>re- Disjonval, was able so accurately to observe the spiders in his cell as to make the observation the basis for his famous ”Aran'>ologie.” Aubert tells of his having had to stay in a room so dark as to make it necessary for others to feel their way, but nevertheless being able to read books without detection because the others could not see the books.
How quickly we get used to darkness and how much more we see after a while, is well known. It is also certain that the longer you are in darkness the more you see. You see more at the end of a day than after a few hours, and at the end of a year, still more. The
eye, perhaps, changes in some degree for just this purpose. But a prolonged use of the visual mechanism tends to hypertrophy- or atrophy, as the eyes of deep-sea fishes show. It is well, in any event, to be careful about contradicting the testimonies of patients who have long lived in the dark, concerning what they have seen. The power to see in the dark is so various that without examination, much injustice may be done. Some people see almost nothing at twilight, others see at night as well as cats. And in court these differences must be established and experimentally verified.
The second important element is the innervation of the muscles in consequence of movement merely seen. So Stricker points out, that the sight of a man carrying a heavy load made him feel tension in the muscles involved, and again, when he saw soldiers exercising, he almost was compelled openly to act as they. In every case the muscular innervation followed the visual stimulus.
This may sound improbable but, nevertheless, everybody to some degree does the identical things. And at law the fact may be of importance in cases of a.s.sault and battery. Since I learned it, I have repeatedly observed in such cases, from harmless a.s.sault to murder, that people, although they had not been seen to deal any blows, were often accused of complicity simply because they were making suspicious movements that led to the following inference: ”They stuck their hands into their trousers pocket looking for a knife, clenched their fists, looked as if they were about to jump, swung their hands.” In many such cases it appeared that the suspects were harmless spectators who were simply more obvious in their innervation of the muscles involved in the a.s.sault they were eagerly witnessing. This fact should be well kept in mind; it may relieve many an innocent.
Section 38. (2) Color Vision.
Concerning color vision only a few facts will be mentioned: 1. It will be worth while, first of all, to consider whether color exists. Liebmann holds that if all the people were blind to red, red would not exist; red, i. e., is some cervical phantasy. So are light, sound, warmth, taste, etc. With other senses we have another world. According to Helmholtz, it is senseless to ask whether cinnabar is red as we see it or is only so as an optical illusion. ”The sensation of red is the normal reaction of normally constructed eyes to light reflected from cinnabar. A person blind to red, will see cinnabar as black, or a dark grayish yellow, and this is the correct reaction
for these abnormal eyes. But he needs to know that his eyes are different from those of other people. In itself the sensation is neither more correct nor less correct than any other even though those who can see red are in the great majority. The red color of cinnabar exists as such only in so far as there are eyes which are similar to those of the majority of mankind. As such light reflected from cinnabar may not properly be called red; it is red only for especial kinds of eyes.” This is so unconditionally incorrect that an impartial judge of photography says[1] that everything that normal eyes call violet and blue, is very bright, and everything they call green and red is very dark. The red-blind person will see as equal certain natural reds, greens and gray-yellows, both in intensity and shadow. But on the photograph he will be able to distinguish the differences in brightness caused by these three otherwise identical colors. We may, therefore, a.s.sume that colors possess *objective differences, and that these objective differences are perceived even by persons of normal vision. But whether I am able to sense the same effect in red that another senses, and whether I should not call red blue, if I had the color-vision of another, is as impossible to discover as it is useless. When the question of color is raised, therefore, we will try to discover only whether the person in question has normal color- vision, or what the nature and degree of his abnormality may be.
[1] W. Heinrich: <u:>bersicht der Methoden bei Untersuchung der Farbenwahr. nehmungen. Krakau 1900.
2. It is not unimportant to know whether single tints are recognizable in the distance. There have been several examinations of this fact. Aubert[2] constructed double squares of ten millimeters and determined the angle of vision at which the color as such could be seen. His results were:
COLOR OF THE WHITE BLACK SQUARE BACKGROUND White 39”
Red 1' 43” 59”
Light Green 1' 54” 1' 49”
Dirty Red 3' 27” 1' 23”